Silicon on germanium

ABSTRACT

A monolayer or partial monolayer sequencing processing, such as atomic layer deposition (ALD), can be used to form a semiconductor structure of a silicon film on a germanium substrate. Such structures may be useful in high performance electronic devices. A structure may be formed by deposition of a thin silicon layer on a germanium substrate surface, forming a hafnium oxide dielectric layer, and forming a tantalum nitride electrode. The properties of the dielectric may be varied by replacing the hafnium oxide with another dielectric such as zirconium oxide or titanium oxide.

RELATED APPLICATION

This application is a Continuation of U.S. application Ser. No.13/617,211, filed 14 Sep. 2012, which is a Continuation of U.S.application Ser. No. 12/829,099, filed 1 Jul. 2010, now issued as U.S.Pat. No. 8,269,254, which is a Divisional of U.S. application Ser. No.11/498,576, filed 3 Aug. 2006, now issued as U.S. Pat. No. 7,749,879,all of which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

This application relates generally to semiconductor devices and devicefabrication and, more particularly to semiconductor structures formed ongermanium substrates.

BACKGROUND

The semiconductor device industry has a continuous market driven need toimprove the operational speed of electronic devices. Previously improvedoperational speed has been obtained by scaling the devices to reduce thetransistor size. Smaller transistors result in improved operationalspeed and clock rate, and reduced power requirements in both standby andoperational modes. To reduce transistor size, the thickness of thesilicon dioxide (SiO₂) gate dielectric is reduced in proportion to theshrinkage of the silicon gate length. For example, ametal-oxide-semiconductor field effect transistor (MOSFET) might use a1.5 nm thick SiO₂ gate dielectric for a gate length of less than 100 nm.As such physical scaling continues, various issue may surface, such asshort channel effect and junction leakage, and thin gate dielectrics maybe a potential reliability issue with gate leakage and time dependentdielectric breakdown. Thus, as silicon based transistors scale down,they may be reaching their fundamental physical limitations of thin gatedielectrics, and very short silicon channels. Nevertheless, smaller,lower power consuming, and more reliable integrated circuits (ICs) willlikely be needed for use in products such as processors, mobiletelephones, and memory devices such as dynamic random access memories(DRAMs) in the future. It has been proposed to use a substrate materialhaving higher charge carrier mobility (and thus relatively higher devicespeed) than the presently used silicon substrate, such as germanium.However, germanium has manufacturing issues and reliability concerns. Ithas also been proposed to use gate dielectrics with higher dielectricconstants (k).

The semiconductor industry reliance upon the ability to scale thedimensions of its basic devices, such as the silicon MOSFET, to achieveimproved operational speed and power consumption may have reached aphysical limit. Device scaling includes scaling the gate dielectric,which has primarily been silicon dioxide (SiO₂). A thermally grownamorphous SiO₂ layer on a silicon substrate provides an electrically andthermodynamically stable material interface with superior electricalisolation. However, increased scaling and other requirements inmicroelectronic devices have created reliability issues as the gatedielectric has become thinner. One potential partial solution includesthe use of materials with higher dielectric constants (k) which wouldallow a thicker physical dielectric thickness with the same equivalentelectrical thickness, and thus address the gate leakage and timedependent dielectric breakdown issues.

The use of germanium substrates has been proposed as a partial solutionto the short channel effect and junction leakage issues, since germaniumhas an electron mobility that is about two times higher than in silicon,and a hole mobility that is about four times higher than in silicon.However, substrate surfaces must have a low and repeatable trappedcharge density and surface trap density, and germanium oxide is watersoluble, has high current leakage and low dielectric breakdown voltage.It is thought that the thermal germanium oxide to germanium substrateinterface may be too intrinsically disordered to be stable and provide aconsistent interface state condition required for semiconductormanufacturing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an atomic layer deposition system;

FIG. 2 illustrates a flow diagram for an embodiment of a method to forma semiconductor structure by atomic layer deposition;

FIG. 3 illustrates a transistor formed by an atomic layer deposition,according to an embodiment;

FIG. 4 shows a different transistor formed by an atomic layerdeposition, according to an embodiment;

FIG. 5 is a simplified diagram for a controller coupled to an electronicdevice, according to an embodiment; and

FIG. 6 is a diagram for an electronic system having devices containingan atomic layer deposited structure, according to an embodiment.

DETAILED DESCRIPTION

The following detailed description refers to the accompanying drawingsthat show, by way of illustration, specific aspects and embodiments inwhich the present arrangement may be practiced. These embodiments aredescribed in sufficient detail to enable those skilled in the art topractice the disclosed invention. Other embodiments may be utilized andstructural, logical, and electrical changes may be made withoutdeparting from the scope of the present disclosure. The variousdescribed embodiments are not necessarily mutually exclusive, as someembodiments may be combined with one or more other embodiments to formnew embodiments.

The terms “wafer” and “substrate” used in the following descriptioninclude any structure having an exposed surface with which to form anintegrated circuit (IC) structure. The term “substrate” is understood toinclude semiconductor wafers, and is also used to refer to semiconductorstructures during processing, and may include other layers that havebeen fabricated thereupon. Both wafer and substrate include doped andundoped semiconductors, epitaxial semiconductor layers supported by abase semiconductor or insulator, as well as other semiconductorstructures well known to one skilled in the art. The term “conductor” isunderstood to generally include n-type and p-type semiconductors. Theterm “insulator” or “dielectric” is defined to include any material thatis less electrically conductive than the materials referred to asconductors or as semiconductors.

The term “horizontal” as used in this application is defined as a planeparallel to the conventional plane or surface of a wafer or substrate,regardless of the orientation of the wafer or substrate. The term“vertical” refers to a direction perpendicular to the horizontal asdefined above. Prepositions, such as “on”, “side” (as in “sidewall”),“higher”, “lower”, “over” and “under” are defined with respect to theconventional plane or surface being on the top surface of the wafer orsubstrate, regardless of the orientation of the wafer or substrate. Thefollowing detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present invention is defined onlyby the appended claims, along with the full scope of equivalents towhich such claims are entitled.

The gate dielectric films discussed herein may include an increaseddielectric constant of gate insulator material over the typical 3.9value of silicon dioxide, and the semiconductor substrate may includecomposite and multilayered materials as compared to typical singlecrystal silicon.

First, a general discussion on high dielectric constant (i.e., high k)materials and germanium substrates will be given. A gate dielectric in ametal oxide semiconductor (MOS) transistor has both a physical gatedielectric thickness and an equivalent oxide thickness (t_(eq)). Theequivalent oxide thickness quantifies the electrical properties, such ascapacitance, of the gate dielectric in terms of a representativephysical thickness. t_(eq) is defined as the thickness of a theoreticalsilicon dioxide (SiO₂) layer that would have the same capacitancedensity as a given dielectric, ignoring leakage current and reliabilityconsiderations.

A SiO₂ layer of thickness, t, deposited or thermally grown on a Sisubstrate surface as a gate dielectric will have a t_(eq) larger thanits physical thickness, t. This t_(eq) results from the capacitance inthe surface channel on which the SiO₂ is formed due to the formation ofa depletion/inversion region in the substrate material. Thisdepletion/inversion region can result in t_(eq) being from 3 to 6Angstroms (Å) larger than the SiO₂ thickness, t. Thus, with thesemiconductor industry driving to scale the gate dielectric equivalentoxide thickness to under 10 Å, the physical thickness requirement for aSiO₂ layer used for a gate dielectric would need to be approximately 4to 7 Å. This is very thin, almost a monolayer, of SiO₂ molecules, andmay be very fragile.

Additional requirements for a SiO₂ layer would depend on the gateelectrode used in conjunction with the SiO₂ gate dielectric. Using aconventional polysilicon gate would result in an additional undesirableincrease in t_(eq) for the SiO₂ layer. This additional thickness couldbe eliminated by using a metal gate electrode, which would not have adepletion layer. Typically, heavily doped polysilicon is currently usedin many complementary metal-oxide-semiconductor field effect transistor(CMOS) technologies. These two effects require devices designed with aphysical SiO₂ gate dielectric layer of as little as 4 Å.

Silicon dioxide is used as a gate dielectric, in part, due to itselectrical isolation properties in a SiO₂—Si based structure. Thiselectrical isolation is due to the relatively large band gap of SiO₂(8.9 eV), which makes it a good insulator. Significant reductions inband gap would eliminate a material for use as a gate dielectric.However, as the thickness of a SiO₂ layer decreases, the number ofatomic layers, or monolayers of the material in the thickness decreases.At a certain thickness, the number of monolayers will be sufficientlysmall that the SiO₂ layer will not have a complete arrangement of atomsas found in a thicker (or bulk) layer. As a result of incompleteformation relative to a bulk structure, a thin SiO₂ layer of only one ortwo monolayers may not form a full band gap. The lack of a full band gapin a SiO₂ gate dielectric may cause an effective short between anunderlying conductive silicon channel and an overlying polysilicon gate.This undesirable property sets a limit on the physical thickness towhich a SiO₂ layer can be scaled. The minimum thickness due to thismonolayer effect is thought to be about 7-8 Å. Therefore, forsemiconductor devices to have a t_(eq) less than about 10 Å, otherdielectrics than SiO₂ need to be considered for use as a gatedielectric.

Thermally grown germanium oxides used as dielectrics do not have theabove noted useful properties, and are water soluble, have lowdielectric breakdown voltages, high leakage rates, and poor interfaceproperties. Other dielectrics deposited on germanium substrates alsohave reliability issues.

For a typical dielectric layer used as a gate dielectric, thecapacitance is determined as in a parallel plate capacitance: C=kε₀A/t,where k is the dielectric constant, ε₀ is the permittivity of freespace, A is the area of the capacitor, and t is the thickness of thedielectric. The thickness, t, of a material is related to its t_(eq) fora given capacitance, with SiO₂ having a dielectric constant k_(ox)=3.9,as

t=(k/k _(ox)) t _(eq)=(k/3.9) t _(eq).

Thus, materials with a dielectric constant greater than that of SiO₂will have a physical thickness that can be considerably larger than adesired t_(eq), while providing the desired electrical equivalent oxidethickness. For example, an alternate dielectric material with adielectric constant of 10 could have a thickness of about 25.6 Å toprovide a t_(eq) of 10 Å, not including any depletion/inversion layereffects. Thus, a reduced equivalent oxide thickness for transistors maybe realized by using dielectric materials with higher dielectricconstants than SiO₂.

The thinner equivalent oxide thickness required for lower transistoroperating voltages and smaller transistor dimensions may be found inmany materials, but typical fabricating requirements makes replacingSiO₂ difficult. The microelectronics industry still uses silicon-baseddevices, which requires that the gate dielectric be grown on a siliconsubstrate. During the formation of the dielectric on the silicon layer,there exists the possibility that a small layer of SiO₂, known as anative oxide, may be formed in addition to the desired dielectric due tohigh temperature processing. The result would effectively be adielectric layer consisting of two sub-layers in connection with eachother and with the silicon layer on which the dielectric is formed. Theresulting overall capacitance would be that of two dielectrics inseries. The t_(eq) of the dielectric layer is the sum of the SiO₂thickness and a multiplicative factor of the thickness, t, of thedielectric being formed with a dielectric constant of k, written as

t _(eq) =t _(SiO2)+(k _(ox) /k)t.

Thus, if a SiO₂ layer is unintentionally formed during the gateinsulator process, the t_(eq) is once again substantially limited by alow dielectric constant SiO₂ layer. In the event that a barrier layer isformed between the silicon layer and the gate dielectric in which thebarrier layer prevents the formation of a SiO₂ layer, the t_(eq) wouldbe limited by the layer with the lowest dielectric constant. However,whether a single dielectric layer with a high dielectric constant or abarrier layer with a higher dielectric constant than SiO₂ is employed,the layer directly in contact, or interfacing with the silicon layermust provide a high quality interface to maintain high channel carriermobility. Preventing the formation of an undesirable SiO₂ layer is oneadvantage of using lower temperatures in an atomic layer deposition(ALD) process.

One of the advantages of using SiO₂ as a gate dielectric has been thatthe formation of the SiO₂ layer results in an amorphous gate dielectric.Having an amorphous structure for a gate dielectric provides reducedleakage current associated with grain boundaries in polycrystalline gatedielectrics, which may cause high leakage paths. Additionally, grainsize and orientation changes throughout a polycrystalline gatedielectric may cause variations in the film's dielectric constant, alongwith uniformity and surface topography issues. Typically, materialshaving the advantage of a high dielectric constant relative to SiO₂ alsohave the disadvantage of a crystalline form, at least in a bulkconfiguration. The best candidates for replacing SiO₂ as a gatedielectric are those with high dielectric constant, which can befabricated as a thin layer with an amorphous form. The amorphous natureof the film is another advantage of using the lower depositiontemperatures available in the ALD deposition process.

Another consideration for selecting the material and method for forminga dielectric film concerns the roughness of a dielectric film on asubstrate. Surface roughness of the dielectric film has a significanteffect on the electrical properties of the gate oxide, and the resultingoperating characteristics of the transistor. The leakage current througha physical 1.0 nm gate dielectric may increase by a factor of 10 forevery 0.1 increase in the root-mean-square (RMS) roughness of thedielectric layer, due among other reasons to the increase electricalfield strength found at sharp points or asperities. This is yet anotheradvantage of using ALD processes that provide smooth surfaces comparedto other deposition methods. For example, during a conventionalsputtering deposition process, particles of the material to be depositedbombard the substrate surface at a high energy. When a particle hits thesurface, some particles adhere, and other particles may cause damage.High energy impacts may create pits. The surface of a sputtered layermay be rough due to the rough interface at the substrate.

A dielectric film formed using atomic layer deposition (ALD) will have asubstantially smooth surface relative to other processing techniques,can provide for controlling transitions between material layers, andhave tight control of thickness and uniformity. As a result of suchcontrol, atomic layer deposited dielectric film may have an engineeredtransition with a substrate surface, or may be formed with many thinlayers of different dielectric materials to enable selection of thedielectric constant and other material properties to a value betweenthat available from pure dielectric compounds.

In addition to the use of high k dielectric materials for the gatedielectric, the semiconductor substrate material may be changed fromsilicon to germanium in order to obtain greatly improved conductionspeed and increased performance. Germanium has about twice the electronmobility and four times the hole mobility of silicon. But the poorsurface interface found in either thermally grown or depositeddielectrics on the germanium surface have been an issue. The use of agermanium oxynitride (GeO_(X)N_(Y)), and other nitridation processes,using ammonia (NH₃) or other nitrogen sources, to improve the interfacestates and reduce native germanium oxidation has not resulted in a fullypassivated surface, and has been found to induce positive fixed surfacecharges as high as 6.8×10¹² charges/cm². The use of a thin silicon layerdirectly on the germanium substrate permits the growth or deposition ofvarious dielectrics without compromising the interface or the improvedcharge mobility obtained with germanium substrates. The thin siliconlayer may have as few as a single monolayer of silicon atoms, but isbeneficially uniform in thickness and composition across at least theMOS channel region. Such a level of uniformity in composition andthickness may be found in ALD reactors, which will now be discussed.

A general discussion of atomic layer deposition (ALD) will be given todescribe how the use of ALD improves the individual layer propertiesdiscussed above and improves manufacturability and process control. ALD,which may also be known as atomic layer epitaxy (ALE), may be consideredas a modification of chemical vapor deposition (CVD), and may also becalled “alternatively pulsed-CVD.” In ALD, precursors are introduced oneat a time to the substrate surface mounted within a reaction chamber (orreactor). The precursors may be a gas, or a sublimated solid, or avaporized liquid, or an atomized liquid, among numerous possible formsof chemical precursors. This introduction of the precursors takes theform of sequential pulses of each precursor. In a pulse of a precursor,the precursor is made to flow into a specific area or region for a shortperiod of time. Between the pulses, the reaction chamber is purged witha gas, which in many cases is an inert gas, and/or evacuated.

In the first reaction step of the ALD process the first precursorsaturates and is chemisorbed (or adsorbed) at the substrate surface,during the first pulsing phase. Subsequent pulsing with a purging gasremoves excess precursor from the reaction chamber, specifically theprecursor that has not been chemisorbed on to the surface.

The second pulsing phase introduces a second precursor to the substratewhere the growth reaction of the desired film takes place, with areaction thickness that depends upon the amount of the chemisorbed firstprecursor. Subsequent to the film growth reaction, reaction byproductsand excess precursor material are purged from the reaction chamber. Witha precursor chemistry where the precursors adsorb and aggressively reactwith each other on the substrate, one ALD cycle can be performed in lessthan one second in properly designed flow type reaction chambers.Typically, precursor pulse times range from about 0.5 sec to about 2 to3 seconds.

In ALD processes, the saturation of all the reaction and purging phasesmakes the film growth self-limiting, which results in large areauniformity in composition and thickness, and superior conformality toother methods. ALD provides controlled film thickness in astraightforward manner by controlling the number of growth cycles. Eachgrowth cycle may produce a single monolayer.

The precursors used in an ALD process may be gaseous, liquid or solid.However, liquid or solid precursors should be volatile with high vaporpressures or low sublimation temperatures. The vapor pressure should behigh enough for effective mass transportation. In addition, solid andsome liquid precursors may need to be heated inside the reaction chamberand introduced through heated tubes to the substrates. The necessaryvapor pressure should be reached at a temperature below the substratetemperature to avoid condensation of the precursors on the substrate.Due to the self-limiting growth mechanisms of ALD, relatively low vaporpressure solid precursors may be used, though evaporation rates may varysomewhat during the process because of changes in surface area of thesolid precursor.

Other desirable characteristics for ALD precursors include thermalstability at the substrate deposition temperature, since precursordecomposition may destroy surface control, which relies on the reactionof the precursor at the substrate surface. A slight decomposition, ifslow compared to the ALD growth rate, may be tolerated. The precursorsshould chemisorb on, or react with the surface, though the interactionbetween the precursor and the surface as well as the mechanism for theadsorption is different for different precursors. The molecules at thesubstrate surface should react aggressively with the second precursor,which for convenience may be called a reactant, to form the desiredfilm. Additionally, precursors should not react with the film to causeetching, and precursors should not dissolve in the film. The ability touse highly reactive chemical precursors in ALD may contrast with theprecursors for conventional metallo-organic CVD (MOCVD) type reactions.Further, the by-products of the reaction should be gaseous in order toallow their easy removal from the reaction chamber during a purge stage.Finally, the by-products should not react or adsorb on the surface.

In an ALD process, the self-limiting process sequence involvessequential surface chemical reactions. ALD relies on chemistry between areactive surface and a reactive molecular precursor. In an ALD process,molecular precursors are pulsed into the ALD reaction chamberseparately. The precursor reaction at the substrate is typicallyfollowed by an inert gas pulse (or purge) to remove excess precursor andby-products from the reaction chamber prior to an input pulse of thenext precursor of the fabrication sequence.

By the use of ALD processes, films can be layered in equal meteredsequences that are all identical in chemical kinetics, depositionthickness per cycle, and composition. ALD sequences generally depositless than a full layer per cycle. Typically, a deposition or growth rateof about 0.25 to about 2.00 Å per ALD cycle can be realized.

The advantages of ALD include continuity at an interface and avoidingpoorly defined nucleating regions, which are typically found in otherthin film deposition methods, for example, chemical vapor depositions(<20 Å) or physical vapor depositions (<50 Å) film. Other advantages ofALD depositions include conformality over a variety of substratetopologies, due to its layer-by-layer deposition technique, use of lowdeposition temperature and mildly oxidizing processes, lack ofdependence on the reaction chamber conditions, growth thicknessdependent solely on the number of cycles performed, and ability toengineer multilayer laminate films with resolution of one to twomonolayers. ALD processes allow deposition control on the order ofsingle monolayers and the ability to deposit monolayers of amorphousfilms.

A cycle of an ALD deposition sequence includes pulsing a precursormaterial, pulsing a purging gas for the precursor, pulsing a reactantprecursor, and pulsing the reactant's purging gas, resulting in a veryconsistent deposition thickness that depends upon the amount of thefirst precursor that chemically adsorbs onto, and saturates, thesurface. This cycle may be repeated until the desired thickness isachieved in a single material dielectric layer, or may be alternatedwith pulsing a third precursor material, pulsing a purging gas for thethird precursor, pulsing a fourth reactant precursor, and pulsing thereactant's purging gas. There need not be a reactant gas if theprecursor can interact with the substrate directly, as in the case of adopant metal layer on a dielectric layer. In the case where thethickness of the first series of ALD cycles results in a dielectriclayer that is only a few molecular layers thick, and the second seriesof cycles also results in a different dielectric layer that is only afew molecular layers thick, this may be known as a nanolayer material,or a nanolaminate. A nanolaminate means a composite film of ultra-thinlayers of two or more different materials in a layered stack, where thelayers are alternating layers of different materials having a thicknesson the order of a nanometer, and may be a continuous film only a singlemonolayer thick of the material. The nanolayers are not limited toalternating single layers of each material, but may include havingseveral layers of one material alternating with a single layer of theother material, to obtain a desired ratio of the two or more materials.Such an arrangement may for example, obtain a dielectric constant thatis between the values of the two or more materials singly. Ananolaminate may also include having several layers of one materialformed by an ALD reaction either over or under a single layer of adifferent material formed by another type of reaction, such as a MOCVDreaction. The layers of different materials may remain separate afterdeposition, or they may react with each other to form an alloy layer.The alloy layer may be viewed as a doping layer, and the properties ofthe dielectric layer may be varied by such doping.

The above described ALD method is used for forming a thin uniform filmof silicon as an interlayer on a germanium substrate. The use ofgermanium as the substrate material allows much faster operation, whilethe silicon interlayer provides a stable interface. To obtain a furtherimprovement in operational speed, the illustrative embodiments use ahigh k dielectric material as the gate dielectric on the siliconinterlayer, but the embodiments are not so limited, and a low k materialmay be used. An illustrative embodiment uses hafnium dioxide (HfO₂) asthe high k gate dielectric material. Alternately, zirconium or titaniumoxides may be used, or other well known high k dielectrics such as STO,BTO, barium oxide, strontium oxide, or combinations and nanolaminates ofhafnium and zirconium oxides, and others. The illustrative embodimentsinclude the use of a tantalum nitride gate electrode layer, but theembodiments are not so limited. Any conductive material may be used forthe gate electrode, or combinations of materials or what may be known asstack gates.

In an illustrative embodiment, a (100) oriented n-type germaniumsubstrate is doped with antimony (Sb) at around 5×10¹⁶ cm⁻³, to form an-type wafer. The substrate is cleaning with a dilute hydrofluoric (HF)acid wash, for example 50 parts water to one part HF, to remove anynative oxide (GeO₂) or contaminants, and rinsed in deionized water (DI).A silicon interlayer is deposited on the substrate by ALD at a substratetemperature of 580° C. with a precursor of silane gas (SiH₄) and anargon (Ar) purge gas. The silicon interlayer may be further stabilizedby annealing in an ammonia (NH₃) ambient. The anneal may be preformedusing a rapid thermal anneal (RTA) using intense light, laser, or otherenergetic means, for example two minutes at 550° C., or equivalent heatcycle. The anneal may form a more stable interfacial layer and preventoxidation of the silicon interlayer, which as noted previously, mayadversely affect the effective capacitance of the gate dielectric.

A hafnium oxide (HfO₂) film is formed on the silicon interlayer usingsequential atomic layer deposition (ALD) at 400° C. An embodimentincludes forming the hafnium oxide layer using a precursor gas such ashafnium tetra-chloride, having a chemical formula of HfCl₄ and watervapor. Other precursors may be used such as anhydrous hafnium nitrate(Hf(NO₃)₄) and water vapor at 160° C. Yet other precursors may includehafnium containing compounds such as tetraisoproxide, or a diketonatechelate precursor gas such as tetramethyl heptanedione ordipivaloylmethane, bis(pentamethyl-cyclopentadienyl),bis(triisopropyi-cyclopentadienyl), ozone, oxygen, nitrous oxide andhydrogen peroxide. Other solid or liquid precursors may be used in anappropriately designed reaction chamber. The use of such precursors inan ALD reaction chamber may result in lower deposition temperatures inthe range of 180° C. to 940° C., more preferably 325° C. to 425° C.Purge gases may include nitrogen, helium, argon or neon. The hafniumoxide films formed may have good thermal and electrical properties, witha high dielectric constant compared to the 3.9 of silicon dioxide. Suchfilms may survive high temperature anneals (sometimes used to reducefixed surface state charges and improve metal to semiconductorresistance) of up to 1000° C. (more preferably 500° C.), and have lowleakage currents of less than 2×10⁻⁷ A/cm² at electric field strengthsof greater than one MVolt/cm.

A gate electrode layer of TaN, in an embodiment 150 nanometer (nm) inthickness is formed by ALD at 325° C. using an illustrative precursor ofTa(OC₂H₅)₅, which may be evaporated from an open topped container heldat 105° C., and a reactant material of water vapor. Such an arrangementallows for a thermally and electrically stable, highly reproducible gateelectrode and dielectric.

FIG. 1 shows an embodiment of an atomic layer deposition system 100 forforming a silicon interlayer on a germanium substrate, a high kdielectric film and a titanium nitride gate electrode. The elementsdepicted permit those skilled in the art to practice the disclosedembodiments without undue experimentation. In FIG. 1, a substrate 108,germanium in an embodiment, on a heating element/wafer holder 106 islocated inside a reaction chamber 102 of ALD system 100. The heatingelement 106 is thermally coupled to substrate 108 to control thesubstrate temperature. A gas distribution fixture 110 introducesprecursor, reactant and purge materials to the substrate 108 in auniform fashion. The materials may be gases, vaporized or atomizedliquids, or sublimated solids, but may be referred to as gases hereinfor simplicity. The gas distribution fixture, sometimes referred to as ashowerhead, allows the precursor materials to react with the substrate108. Excess gas and reaction products are removed from chamber 102 byvacuum pump 104 through control valve 105. Each gas originates fromindividual gas sources 114, 118, 122, 126, 130, and 134, with a flowrate and time controlled by mass-flow controllers 116, 120, 124, 128,132 and 136. Gas sources 126, 130 and 134 provide a variety of precursorgases, either by storing the precursor as a gas or by providing forevaporating a solid or liquid material to form the selected precursorgas.

Also included in the system is purging gas source 114, coupled tomass-flow controller 116. The embodiment may use only one purge gas forall disclosed illustrative purging steps, or additional purge gassources may be added in any desired number, such that in the generalcase any purge gas may be used in any particular step, or anycombination of purge gases may be used simultaneously, or alternatingthe purge gases as required for the particular desired result.Additional purging gas sources can be constructed in ALD system 100, forexample, one purging gas source for each different precursor andreactant gas. For a process that uses the same purging gas for multipleprecursor gases, fewer purging gas sources may be required for ALDsystem 100, as in the current illustrative embodiment.

The precursor gas used may be a combination of various precursors toform a combination layer. The precursor, reactant and purge gas sourcesare coupled by their associated mass-flow controllers (116, 120, 124,128, 132 and 136) to a common gas line, or conduit 112, which is coupledto the gas-distribution fixture 110 inside the reaction chamber 102. Gasconduit 112 may also be coupled to an additional vacuum pump, or exhaustpump (not shown), to remove excess precursor gases, purging gases, andby-product gases at the end of a purging sequence from the gas conduit112.

Vacuum pump, or exhaust pump, 104 is coupled to chamber 102 by controlvalve 105, which may be a mass-flow valve, to remove excess precursorgases, purging gases, and by-product gases from reaction chamber 102 atthe end of a purging sequence. It is also possible to control theeffective precursor gas pumping rate by means of what may be known asballasting the system, by means of an inert gas input somewhere in thesystem, for example in the vacuum pump 104. For convenience, controldisplays, mounting apparatus, temperature sensing devices, substratemaneuvering apparatus, and necessary electrical connections as are knownto those skilled in the art are not shown in FIG. 1. Though ALD system100 is well suited for practicing the present embodiment, othercommercially available ALD systems may also be used.

The use and operation of reaction chambers for deposition of films areunderstood by those of ordinary skill in the art of semiconductorfabrication. The disclosed embodiments may be practiced on a variety ofsuch reaction chambers without undue experimentation. Furthermore, oneof ordinary skill in the art will comprehend the necessary detection,measurement, and control techniques in the art of semiconductorfabrication upon reading the disclosure. The elements of ALD system 100may be controlled by a computer using a computer readable medium tocontrol the individual elements such as pressure control, temperaturecontrol, and gas flow within ALD system 100. To focus on the use of ALDsystem 100 in the various disclosed embodiments, the computer is notshown. Those skilled in the art can appreciate that system 100 may beunder computer control.

FIG. 2 illustrates a flow diagram of operational steps for an embodimentof a method to form a series of ALD layers having an illustrative threedifferent materials: silicon, hafnium oxide and titanium nitride. Eachof the three different materials, which in the general case each layer,may be a combination of materials. At 202, a substrate is prepared toreact immediately with, and chemisorb the first precursor gas, orcombination of precursor gases. This preparation will removecontaminants such as thin organic films, dirt, and native oxide from thesurface of the substrate, and may include a hydrofluoric acid rinse, orsputter etching in the reaction chamber 102. At 204 a first precursormaterial, or combination of precursor materials, enters the reactionchamber for a predetermined length of time, for example 0.5-2.0 seconds.The first precursor material is chemically adsorbed onto the surface ofthe substrate, the amount depending upon the temperature of thesubstrate, in one embodiment 580° C. for silane and argon for a siliconinterlayer, or 400° C. hafnium tetrachloride for hafnium oxide, or 325°C. with Ta(OC₂H₅)₅ for the TaN layer. In each case the temperature andthe presence of sufficient flow of the precursor materials controls theamount of adsorbed material, and thus the layer thickness for thatcycle. In addition, the pulsing of the precursor may use a periodproviding uniform coverage of an adsorbed monolayer on the substratesurface, or may use a period that provides partial formation of amonolayer on the substrate surface.

At 206 a first purge gas enters the reaction chamber for a predeterminedlength of time sufficient to remove substantially all of thenon-chemisorbed first precursor materials. Typical times may be 1.0-2.0seconds, with a purge gas comprising nitrogen, argon, neon, combinationsthereof, or other gases such as hydrogen. At 208 a first reactant gasmay in the general case enter the chamber for a predetermined length oftime, sufficient to provide enough of the reactant to chemically combinewith the amount of chemisorbed first precursor material on the surfaceof the substrate. In the present embodiment of forming a siliconinterlayer no reactant may be needed depending upon the substratetemperature, or a nitrogen gas flow may be used. Typical reactantmaterials may include mildly oxidizing materials such as water vapor,but may in general also include hydrogen peroxide, various nitrogenoxides such as nitrous oxide, ozone and oxygen gas, and combinationsthereof. It should be noted that the difference between a precursor anda reactant is basically the timing of the introduction of the materialinto the reaction chamber. At 210 a second purge gas, which may be thesame or different from the first purge gas, enters the chamber for apredetermined length of time, sufficient to remove substantially allnon-reacted materials and any reaction byproducts from the chamber.

At 212 a decision is made as to whether or not the thickness of thefirst material has reached the desired thickness, in an embodiment theinterlayer of silicon, or whether another deposition cycle is requiredof the first material. If another deposition cycle is needed, then theoperation returns to 204, until the desired first layer is completed, atwhich time the process moves on to the deposition of the second materialat 214.

At 214 a second precursor material, or combination of precursormaterials, enters the reaction chamber for a predetermined length oftime, typically 0.5-2.0 seconds. The second precursor material ischemically adsorbed onto the surface of the substrate, in this case thetop surface of the first material, the amount of absorption dependingupon the temperature of the substrate, and the presence of sufficientflow of the precursor material. In addition, the pulsing of theprecursor may use a period that provides uniform coverage of an adsorbedmonolayer on the substrate surface, or may use a period that providespartial formation of a monolayer on the substrate.

At 216 the first purge gas is shown as entering the chamber, but theinvention is not so limited. The purge gas used in the second materialdeposition, in an embodiment hafnium oxide, may be the same or differentfrom either of the two previously noted purge gases, and FIG. 1 could beshown as having more than the two purge gases illustrated. The purgecycle continues for a predetermined length of time sufficient to removesubstantially all of the non-chemisorbed second precursor material.

At 218 an illustrative second reactant gas, which may be the same ordifferent from the first reactant gas, enters the chamber for apredetermined length of time, sufficient to provide enough of thereactant to chemically combine with the amount of chemisorbed secondprecursor material on the surface of the substrate. In an embodiment thereactant is water vapor. In certain general cases there may be no secondreactant gas, and the precursor chemically reacts with the firstmaterial to form an alloy or a doped layer of the first material. At 220a purge gas enters the chamber, which may be the same or different fromany of the previously discussed purge gases, for a predetermined lengthof time, sufficient to remove substantially all non-reacted materialsand any reaction byproducts from the chamber.

At 222 a decision is made as to whether or not the thickness of thesecond material has reached the desired thickness, or whether anotherdeposition cycle is required. If another deposition cycle is needed,then the operation returns to 214, until the desired second layer iscompleted. The desired thicknesses of the first and second materials inthe dielectric may not be the same thickness, and there may be moredeposition cycles for one material as compared to the other. Thethickness of each of the first two layers may be selected to obtain adesired final composition, in an embodiment a thick enough layer ofsilicon to support a gate dielectric layer of hafnium oxide with goodinterface properties. If the second layer has reached the desiredthickness, the process moves on to a decision at 224 of whether thenumber of layers of the first and second materials has reached thedesired number. In this illustrative embodiment a single layer of thefirst material and a single layer of the second material have beencompleted at this point in the process, which in the currently describedembodiment are the desired interlayer of silicon and a dielectric layerof a desired number of hafnium oxide layers. In the more general case,if more than a single layer of each material is desired, the processmoves back to another deposition of the first dielectric material at204. After the number of interleaved layers of materials one and two hasreached the desired value, the deposition process goes to the thirdlayer deposition at 226 in this illustrative embodiment. In the generalcase, the gate electrode material may be deposited by ALD in a differentreactor, or may be deposited by MOCVD or other deposition method. In theembodiment the third deposition is tantalum nitride, and the depositionoccurs as described previously for the first and second layers.

At 226 the third precursor enters the chamber for a specified period oftime sufficient to saturate the surface. In an embodiment the thirdprecursor is Ta(OC₂H₅)₅, which is a liquid, and thus may be evaporated,or entrained in a carrier gas flow via a bubbler. At 228 a purge gasenters the chamber, followed at 230 by the third reactant, in anembodiment water vapor. At 232 a purge gas removes the reaction productsand un-reacted precursors and reactants. At 234 the number of cycles ofTiN growth is compared to the desired thickness, and the process returnsto 226 if another layer is required, or ends at 236 if the finalthickness has been reached via the correct number of ALD cycles.

The embodiments described herein provide a process for growing a siliconinterlayer on a germanium substrate by ALD, a high k dielectric layer onthe silicon interlayer by ALD, and a tantalum nitride gate electrode byALD, but the embodiments are not so limited, and may include non ALDdepositions. The described materials and process may be implemented toform transistors, capacitors, non volatile memory devices,micro-electro-mechanical devices (MEMs) and other electronic systemsincluding information handling devices. The disclosed embodiments arenot limited to three different materials as in the disclosedembodiments, and the equipment described in FIG. 1 could include more orfewer precursor and reactant materials, which are not described forsimplicity.

FIG. 3 illustrates a single transistor 300 in an embodiment of a methodto form a metal oxide semiconductor field effect transistor (MOSFET)containing an ALD deposited silicon interlayer, a high k gate oxidelayer and a TaN gate electrode. This embodiment may be implemented withthe system 100 of FIG. 1 used as an atomic layer deposition system. Asubstrate 302 is prepared, in an embodiment a germanium substrate,including cleaning substrate 302 and forming various layers and regionsof the substrate, such as drain diffusion 304 and source diffusion 306of an illustrative GeSi metal oxide semiconductor (MOS) transistor 300,which may occur prior to forming a gate dielectric, or after forming thegate dielectric and gate electrode, using the well known self aligningmethod. The substrate may be cleaned to provide an initial substratedepleted of its native oxide to avoid having the above describedsituation of two capacitors in series. The initial substrate may becleaned to provide a hydrogen-terminated surface to avoid potentialsurface state traps and trapped charges. The substrate may undergo afinal hydrofluoric (HF) rinse prior to ALD processing to provide thesilicon substrate with a hydrogen-terminated surface without a nativeoxide layer. Cleaning immediately preceding atomic layer deposition aidsin reducing an occurrence of an oxide as an interface between thegermanium based substrate and the silicon interlayer, or between theinterlayer and the gate dielectric formed using the atomic layerdeposition process. The sequencing of the formation of the regions ofthe transistor being processed may follow the generally understoodfabrication of a MOS transistor, as is well known to those skilled inthe art.

The silicon interlayer 308 covering the area on the substrate 302between the source and drain diffused regions 304 and 306 may bedeposited by ALD in this illustrative embodiment, and may comprise oneor more silicon layers to obtain a desired silicon thickness. Thesilicon interlayer may be formed of multiple silicon monolayers, and mayhave any desired thickness to obtain a desired interface state densitybetween the layer 308 and the germanium substrate 302. The siliconinterlayer 308 may have an extent greater than the distance between thesource 304 and the drain 306, as shown in the figure, and in general maytypically extend over the entirety of the surface of the germaniumsubstrate (or wafer).

A gate dielectric 310, in an embodiment a high k dielectric such ashafnium oxide, is formed on the silicon interlayer, typically withoutremoving the substrate from the vacuum chamber in which the siliconinterlayer was grown. This may be done to eliminate potentialcontamination and oxidation sources. In an embodiment, the ALDdeposition includes a precursor of anhydrous hafnium nitride and areactant of water vapor at a substrate temperature of from 160° C. to250° C. A single ALD cycle may result in a layer of from 1-14 Å perdeposition cycle, depending upon the deposition parameters used such asprecursor, temperature, flow rate and reactant. To obtain an effectivegate dielectric that is electrically equivalent to a 5 Å silicon dioxidelayer, the hafnium oxide layer, which may have a dielectric constant ofabout 12, could be as thick as 15 Å, and depending upon the exacttemperature and precursor chemistry, may represent from 2 to 15 ALDcycles, each taking as few as 1-2 minutes.

There may be a diffusion barrier layer (not shown for simplicity)inserted between the dielectric layer 310 and the silicon interlayer 308on the substrate 302 to prevent metal contamination from the metalliccomponent of the gate oxide (hafnium in this illustrative embodiment)from affecting the electrical properties of the device. The transistor300 has a conductive material forming a gate electrode 318, in thisillustrative embodiment formed of ALD layers of tantalum nitride, formedby use of a Ta (OC₂H₅) ₅ precursor, and a water vapor reactant at 325°C. However, any conductive media may be used for the gate electrode, andthe conductive media may be selected to obtain a specific work function.The conductive material may be polysilicon, various metals, refractorymetals, or metal silicides.

As illustrative embodiments, the described structure may be used as asimple transistor having high carrier mobility due to the germaniumsubstrate, a controllable and relatively high dielectric constant gatedielectric, and a stable gate electrode having a desired work function.Similar structures may also be used as a non volatile memory device, asa capacitor, or as a tunnel gate insulator, or as a floating gatedielectric in a flash memory device. Use of the described structures isnot limited to germanium substrates, but may be used with a variety ofother semiconductor substrates, such as by forming a germanium layer ona silicon substrate, then forming the silicon interlayer 308 on thegermanium layer. Other embodiments of the described structure may alsoinclude devices used in various integrated circuits, memory devices, andelectronic systems.

FIG. 4 illustrates an alternative transistor 400, having combinations oftwo or more gate dielectric materials, including the use of interleavedlayers of varying thickness for the gate dielectric 310 of FIG. 3. Thislaminated dielectric formed of layers of different materials may bereferred to as the gate oxide, and while shown as individual distinctlayers for clarity, is typically effective either as a single alloyedlayer, or a substituted layer, or a doped layer.

In an embodiment including a transistor 400, the germanium substrate402, has source and drain regions 404 and 406 formed either before orafter the gate electrode 418 is formed. The gate dielectric is formed ofa laminated dielectric having interleaved layers 410-416 of twodifferent materials, for example hafnium oxide 410 and 414, and titaniumoxide 412 and 416. The sequencing and thickness of the individual layersof the gate dielectric may depend upon the application and may include asingle layer of each material, one layer of one of the materials andmultiple layers of the other, or other combinations of layers includingdifferent layer thicknesses. By selecting thickness and composition ofeach layer, a nanolaminate structure can be engineered to have apredetermined dielectric constant and composition. Although the materiallayers are shown in this illustrative example as being distinct layers,the oxide may be alloyed to form a single material layer having thedesired dielectric constant. The illustrative embodiment shows thedielectric layers 410, 412, 414 and 416 having the same thickness;however the desired dielectric properties of the nanolaminate film maybe best achieved by adjusting the ratio of the thickness of thedielectric layers to different values. For example it may be desired tohave the dielectric constant value change in the dielectric near thesilicon interlayer 408 and the substrate 402 versus near the gateelectrode 418. This may be known as a graded dielectric. Transistors,capacitors, and other electronic devices having the structures shown inFIGS. 3 and 4 may be used in memory devices and electronic systemsincluding information handling devices such as wireless systems,telecommunication systems, computers and integrated circuits.

FIG. 5 illustrates a diagram for an electronic system 500 having one ormore devices having a germanium transistor containing an atomic layerdeposited silicon interlayer, gate insulator and gate electrode formedaccording to various embodiments. Electronic system 500 includes acontroller 502, a bus 504, and an electronic device 506, where bus 504provides electrical conductivity between controller 502 and electronicdevice 506. In various embodiments, controller 502 and/or electronicdevice 506 include an embodiment for a dielectric layer containinggermanium transistors formed by ALD layers as previously discussedherein. Electronic system 500 may include, but is not limited to,information handling devices, wireless systems, telecommunicationsystems, fiber optic systems, electro-optic systems, and computers.

FIG. 6 depicts a diagram of an embodiment of a system 600 having acontroller 602 and a memory 606. Controller 602 and/or memory 606include an ALD formed germanium transistor with a silicon interlayer inaccordance with the disclosure. System 600 also includes an electronicapparatus 608, and a bus 604, where bus 604 may provide electricalconductivity and data transmission between controller 602 and electronicapparatus 608, and between controller 602 and memory 606. Bus 604 mayinclude an address, a data bus, and a control bus, each independentlyconfigured. Bus 604 also uses common conductive lines for providingaddress, data, and/or control, the use of which may be regulated bycontroller 602. In an embodiment, electronic apparatus 608 includesadditional memory devices configured similarly to memory 606. Anembodiment includes an additional peripheral device or devices 610coupled to bus 604. In an embodiment controller 602 is a processor. Anyof controller 602, memory 606, bus 604, electronic apparatus 608, andperipheral devices 610 may include an ALD formed germanium transistorwith a silicon interlayer, in accordance with the disclosed embodiments.

System 600 may include, but is not limited to, information handlingdevices, telecommunication systems, computers, cameras, phones, wirelessdevices, displays, chip sets, set top boxes, games and vehicles.Peripheral devices 610 may include displays, additional storage memory,or other control devices that may operate in conjunction with controller602 and/or memory 606. It will be understood that embodiments areequally applicable to any size and type of memory circuit and are notintended to be limited to a particular type of memory device. Memorytypes include a DRAM, SRAM (Static Random Access Memory) or Flashmemories. Additionally, the DRAM could be a synchronous DRAM commonlyreferred to as SGRAM (Synchronous Graphics Random Access Memory), SDRAM(Synchronous Dynamic Random Access Memory), SDRAM II, and DDR SDRAM(Double Data Rate SDRAM), as well as other emerging DRAM technologies.

The disclosed embodiments include a method for making devices by forminga silicon layer on a single crystal germanium semiconductor substrate byatomic layer deposition at a predetermined temperature, forming adielectric layer on the silicon layer; and forming an electricallyconductive layer on the dielectric layer. The atomic layer depositionprocess includes exposing a surface at a preselected temperature to aprecursor material for a preselected time period and a preselected flowvolume of the precursor material to saturate the substrate surface withthe precursor material as a first step. Then exposing the surface to apreselected volume of a purge material for a preselected time period toremove substantially all of a non-adsorbed portion of the precursormaterial from the surface, to prepare for the second step. Then exposingthe surface to a preselected volume of a reactant material for apreselected time period to react with the adsorbed portion of theprecursor material on the substrate surface to form a material layerhaving a first intermediate thickness, which essentially completes afirst deposition cycle. Finally exposing the substrate surface to apreselected volume of a purge material for a preselected time period toremove substantially all of a non-reacted portion of the reactantmaterial, and a plurality of reaction byproducts from the surface, andrepeating the first deposition cycle until a final dielectric materialthickness is obtained for the material layer.

Typical precursors include hafnium tetrachloride, hafnium tetraiodide,hafnium nitride, silane, and Ta(OC₂H₅)₅, which react with reactants suchas water vapor, nitrous oxide, hydrogen peroxide, ozone and oxygen attemperatures of from 325 to 650° C. to form the monolayers of thematerial. Typical purge materials include inert gases such as argon,neon, helium, and gases that do not react at the deposition conditionssuch as nitrogen and hydrogen.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement that is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. This application isintended to cover any adaptations or variations of embodiments of thepresent invention. It is to be understood that the above description isintended to be illustrative, and not restrictive, and that thephraseology or terminology employed herein is for the purpose ofdescription and not of limitation. Combinations of the above embodimentsand other embodiments will be apparent to those of skill in the art uponstudying the above description. The scope of the present inventionincludes any other applications in which embodiments of the abovestructures and fabrication methods are used. The scope of theembodiments of the present invention should be determined with referenceto the appended claims, along with the full scope of equivalents towhich such claims are entitled.

What is claimed is:
 1. An electronic device comprising: at least onetransistor containing an atomic layer deposited layer of silicon, anatomic layer deposited layer of a metal oxide in an integrated circuit;and a conductive layer contacting the dielectric layer.